Complete automation of molded case circuit breaker components by robotic assembly has not heretofore been completely successful. One impediment to complete robotic assembly is the attachment of a flexible conductive braid between the circuit breaker contact and the circuit breaker load terminal lug.
Early attempts to eliminate the electrical contact braid are found in U.S. Pat. Nos. 3,023,292, 3,033,964 and 3,073,936 wherein a pair of contact arms are supported on a contact arm support by means of a pivot pin and a thick spring clip is fastened to the contact arm support and arranged around both the contact arms and the support. Direct electrical connection between the spring clip and the terminal conductor in some industrial-rated circuit breaker designs advantageously improves the electrical conduction between the terminal conductor and the movable contact arm by the electromagnetic forces of attraction generated by the current through the spring clip. The increasing electric current increases the electromagnetic force on the juncture between the movable contact arm and the terminal conductor to create an increasing compressive force therebetween.
In lower ampere-rated current limiting industrial circuit breakers, the forces exerted by the spring clip on the movable contact arm and the terminal conductor must remain relatively constant with increasing current to ensure that the contacts can be electrodynamically repulsed and separated under high current faults such as those occurring with short circuits. The contact arm must rapidly move about its pivot in the early stages of the current wave-form to separate the contacts with minimum let-through current at the instant of separation. This is not easily obtained when the compressive forces on the movable contact arm and the terminal conductor substantially increase at the time the movable contact arm is required to rotate about its pivot.
U.S. Pat. Nos. 4,240,053 and 4,554,427 each disclose circular segments formed within the movable contact arm that are arranged over a respective circular segment formed on the terminal conductor to form a conductive junction between the contact arm and the terminal conductor.
U.S. Pat. No. 4,160,142 utilizes a pair of washers, a nut and a bolt to connect the movable contact arm to the terminal conductor to electrically connect the contact arm with the terminal conductor.
U.S. Pat. No. 4,245,203 discloses a clinch type electrical connection between the movable contact arm and a pair of bifurcated upright posts. Clamping force upon the contact arm pivot is provided by the resilience of the posts and by a bias spring clip.
A more recent design that enables a braidless movable contact arm that is robotically assembled is described in U.S. Pat. No. 4,733,033, which Patent is incorporated herein for reference purposes. This Patent discloses the use of a spring having a planar configuration capable of holding the contact arm against its support posts with sufficient force to maintain electrical contact during overcurrent conditions. When this design is used within higher ampere-rated current limiting industrial circuit breakers, a parallel current path should be connected between the movable contact arms and the contact arm support posts to prevent the occurrence of arcing between the contact arm and the support posts under intense short-circuit overcurrent conditions.
One purpose of the instant invention accordingly is to provide a movable contact arm arrangement that includes a parallel current path between the movable contact arm and the contact arm support posts without interfering with the rapid pivotal motion of the movable contact arm during intense short circuit interruption.